The disclosure of Japanese Patent Application No. 2002-225171 filed on Aug. 1, 2002, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a fuel injection system for an internal combustion engine starting time and a control method of same.
2. Description of Related Art
When an internal combustion engine (hereinafter simply referred to as xe2x80x9cenginexe2x80x9d where appropriate) is started and the engine speed subsequently increases, the intake amount which is supplied into engine cylinders decreases and the negative pressure in each of the engine cylinders increases. Namely, as the engine speed increases, the intake amount supplied into the engine cylinders decreases. In view of this, there are known technologies, such as disclosed in Japanese Patent Laid-Open Publication No. 1-173188, in which a fuel injection control is performed so as to reduce the amount of fuel to be injected (hereinafter, referred to as a xe2x80x9cfuel injection amountxe2x80x9d where appropriate) with an increase in the engine speed during engine start.
Not only after the completion of warming-up but also during engine start, when an air-fuel ratio in the engine cylinder is rich, a large amount of unburned HC is generated. When the air-fuel ratio is too lean, conversely, combustion flames do not sufficiently spread, which may also result in the generation of a large amount of unburned HC. Namely, it is necessary to maintain the air-fuel ratio at the stoichiometric air-fuel ratio or at a slightly lean air-fuel ratio so as to suppress the generation of unburned HC.
Meanwhile, if the engine is of a type which directly injects fuel into the cylinder, when fuel is injected during engine start, a large amount of the injected fuel adheres, in liquid form, to a top face of a piston or an inner surface of a cylinder. Also, if the engine is of a type which injects fuel into intake ports, a large amount of the injected fuel adheres, in liquid form, to the inner surface of each intake port. Thus, in either type of internal combustion engine, air-fuel mixtures are formed by only a small part of injected fuel. The fuel adhered on the top face of the piston or on the inner surface of the intake port gradually evaporates to form air-fuel mixtures until the piston reaches a top dead center for compression. This air-fuel mixture accounts for a sizable proportion of the entire air-fuel mixture formed in the engine cylinder. Accordingly, in the aforementioned case, the air fuel ratio of the air-fuel mixture formed in the engine cylinder largely depends on the amount of the fuel evaporated from the inner surface.
The amount of the fuel which evaporates from the inner surface is proportional to the length of time until the piston reaches the vicinity of the top dead center for compression. The shorter this length of time becomes, a smaller amount of the fuel evaporates from the inner surface. Meanwhile, the length of time until the piston reaches the vicinity of the top dead center for compression is inversely proportional to the engine speed. Accordingly, as the engine speed increases, the air-fuel ratio of the air-fuel mixture increases.
As mentioned above, it is necessary to maintain the air-fuel ratio at the stoichiometric air-fuel ratio or at a slightly lean air-fuel ratio in order to suppress the generation of unburned HC. However, as mentioned above, as the engine speed increases, the air-fuel ratio of the air-fuel mixture increases. Accordingly, it is necessary to increase the fuel injection amount as the engine speed increases in order to maintain the air-fuel ratio at the stoichiometric air-fuel ratio or at a slightly lean air-fuel ratio while the engine speed is increasing during engine start. At this time, for suppressing the generation of unburned HC, it is necessary to prevent the air-fuel ratio from being temporarily rich or excessively lean.
As described earlier, in the conventional fuel injection control, when the engine speed is increasing during engine start, the fuel injection amount is reduced. When the fuel injection amount is thus reduced with the increase in the engine speed, the air-fuel ratio gradually increases while largely fluctuating. Therefore, when the engine speed starts to increase, the air-fuel ratio needs to be set to a considerably low ratio, which is usually a rich air-fuel ratio, so that the fuel injection amount can be set so as to prevent the air-fuel ratio from becoming excessively lean when the increase in the engine speed ends, and thereby to avoid misfires. Thus, the air-fuel ratio is made rich, and a large amount of unburned HC is therefore emitted.
As described above, if the fuel injection amount is reduced with an increase in the engine speed during engine start as in the conventional fuel controls, a large amount of unburned HC is generated, although the engine can be started. Namely, since the behavior of actual air-fuel ratios in engine cylinders during the engine start is not sufficiently determined in the conventional injection controls, a large amount of unburned HC is unavoidably generated.
It is an object of the invention to provide a fuel injection system for an internal combustion engine starting time and a control method thereof, which mainly achieve a reduction of unburned HC.
Therefore, according to an exemplary embodiment of the invention, in an internal combustion engine having a plurality of cylinders, there is provided a fuel injection system for an internal combustion engine starting time which sets an amount of fuel that is injected into each cylinder sequentially in a first cycle of the fuel injection during a normal engine start where an engine speed to increases, such that an amount of fuel injected into one of the cylinders in a last injection within the first cycle is larger than an amount of fuel injected into another one of the cylinders in a first injection within the first cycle.
According to a further exemplary embodiment of the invention, there is provided a control method for a fuel injection system for an internal combustion engine starting time having a plurality of cylinders. In this control method, an amount of fuel injected into each cylinder sequentially in a first cycle of fuel injection during a normal engine start in which an engine speed increases is set such that an amount of fuel injected into one of the cylinders in a last injection within the first cycle is larger than an amount of fuel injected into another one of the cylinders in a first injection within the first cycle.
As mentioned above, in order to suppress the generation of unburned HC during engine start, it is desirable to maintain the air-fuel ratio at the stoichiometric air-fuel ratio or at a slightly lean air-fuel ratio. The amount of fuel which evaporates from an inner surface of the cylinder of the internal combustion engine decreases as the engine speed increases. Accordingly, it is desirable to increase the fuel injection amount as the engine speed increases during engine start.
According to the above-mentioned fuel injection system for an internal combustion engine starting time and the control method thereof, the amount of the fuel which is injected into each cylinder sequentially in the first cycle of the fuel injection is set such that the amount of fuel injected into one of the cylinders in the last injection within the first cycle is larger than the amount of fuel injected into another one of the cylinders in the first injection within the first cycle. With this arrangement, it is possible to maintain the air-fuel ratio at the stoichiometric air-fuel ratio or at a slightly lean air-fuel ratio. Therefore, it is possible to suppress the generation of unburned HC during engine start.